Non-Patent Documents 1 and 2 disclose thin film transistors each having an amorphous metal oxide semiconductor layer which has been formed through sputtering or CVD. In the case of formation of such an amorphous metal oxide semiconductor layer by means of a vacuum deposition type film-forming apparatus (e.g., a sputtering apparatus), pattern formation is generally performed through vapor deposition by the mediation of a mask or through photolithographic etching. These conventional techniques, however, have drawbacks, such as high cost when a large-scale substrate is used, and cumbersome manufacturing processes. Amorphous metal oxide semiconductor layers produced through these methods also have drawbacks in that quality varies among semiconductor layer products obtained on the same substrate and between the different substrates, and that high-temperature firing is required to reduce such quality variations.
Recently, Non-Patent Documents 3 to 6 and Patent Documents 1 to 3 have proposed production of thin film transistors which includes formation of a metal oxide semiconductor layer film through a coating technique.
In these production methods based on a coating technique, a metal complex is dissolved in a solvent to thereby form a precursor composition; the precursor composition is applied onto a substrate through a coating technique such as spin coating or ink-jet coating; and the coating is fired, whereby a metal oxide semiconductor layer is formed. However, when a substrate having low heat-resistance, such as a plastic substrate, is used, there occur thermal expansion and contraction or deterioration of the substrate, or decomposition of the substrate itself. In order to prevent such drawbacks, there is demand for a precursor composition which can form a dense amorphous metal oxide semiconductor layer at a low temperature which does not exceed 300° C.
Non-Patent Document 3 discloses a procedure of determining the optimum firing temperature. In this procedure, the pyrolysis behavior of a precursor composition is analyzed in advance through thermal analysis of the composition, whereby a firing temperature at which heat generation or heat absorption, which would otherwise be caused by thermal weight loss and/or thermal reaction, is selected. However, Non-Patent Document 4 discloses that, when such a precursor composition is not fired at a temperature sufficiently higher than the temperature at which the precursor composition undergoes thermal weight loss and/or thermal reaction, impurities remaining in the formed film affect the operational behavior of the resultant device, and the performance of the resultant device is lower than that of the same device employing a metal oxide semiconductor layer produced through vacuum deposition.
Patent Documents 1 and 3 disclose methods for producing a thin film semiconductor layer, the methods employing a low-boiling-point alcohol and water, instead of a high-boiling-point organic solvent, which may produce residual substances. However, a dense amorphous metal oxide semiconductor layer fails to be formed through the aforementioned methods.
Non-Patent Documents 5 and 6 disclose that a dense amorphous film fails to be formed by firing such a coating composition, concomitant with coating failures observed on the surface of the formed film. In order to overcome the drawback, there has been proposed an approach of limiting the ingredient concentration of the composition. However, even when the approach is employed, an amorphous metal oxide semiconductor layer having a sufficient thickness fails to be formed.